This invention relates to a method of recovering vanadium values from aqueous solutions. More particularly, this invention relates to a method of recovering vanadium values from an aqueous ammonium bicarbonate solution.
A number of important hydrometallurgical processes involve the extraction of metal species in the form of anionic metal complexes from aqueous pregnant liquors, subsequent stripping of the metal values from the organic phase into an aqueous phase, and finally, separation of the metal values from the aqueous phase. Such extracting and stripping processes are especially important in the hydrometallurgy of Group VB and Group VIB metals. One particularly important source of aqueous pregnant liquors containing both Group VB and Group VIB metal values is the pregnant liquor produced from leaching spent hydroprocessing catalysts.
The chemistry of Group VB and Group VIB metals in an aqueous solution is particularly complex. Each metal in both Groups forms a variety of oxidation states that can in turn form a variety of oxygen-containing anions in aqueous solution. These metal-oxygen complexes undergo complex reactions in aqueous solution that depend on such variables as pH, temperature, concentration and chemical environment. Each metal of the Group VB and Group VIB undergoes reactions at unique conditions. The complex aqueous chemistry of Group VB and Group VIB metals makes the liquid-liquid phase transfer processes particularly difficult, especially if more than one metal species of Group VB and Group VIB are present.
Crude oils frequently contain metals, particularly iron, nickel, tungsten and vanadium. Spent hydroprocessing catalysts that have been in contact with metals containing crude or residual oil can therefore contain both molybdenum, a frequent catalytic metal, tungsten and vanadium. Extracting and stripping these metal values when any one is present alone or simultaneously when mixtures of the metals are present from a leach liquor from spent catalysts therefore presents a difficult and subtle problem.
It is preferred, that if the metals from catalysts are to be recovered by hydrometallurgical techniques, that the total flow scheme be one that allows maximum recycle of reagents and minimum introduction of extraneous ions. Since one preferred recovery system as described in U.S. Pat. Nos. 4,434,141; 4,434,140; and 4,442,074 involves an aqueous ammoniacal leach using ammonium carbonate or ammonium sulfate, it is preferred that only ammonia, ammonium ion, carbonate or sulfonate ion be added to the aqueous streams. This self-imposed limitation complicates the recovery scheme of Group VB and Group VIB metals even more.
It is possible, at pH values in the range of about 3 to about 5, to extract Group VB and Group VIB metals when any one is present alone or simultaneously when present as mixtures thereof, and particularly, molybdenum, tungsten and vanadium, into an organic phase by use of a quaternary amine as a liquid anion exchange reagent. One such reagent is tri-caprylyl methyl ammonium chloride, which is sold under the trade name Aliquat 336.RTM. by the Henkel Chemical Company.
Aliquat 336.RTM. and related compounds that have extracted constituents from Group VB and Group VIB on them tend to be difficult to strip, especially if the strip is to be chemically compatible with the leach. In U.S. Pat. No. 3,083,085 an aqueous solution of NaCl is used, to strip molybdenum from the organic phase. This requires regeneration of the organic extracting solution with sulfuric acid and the use of expensive alloys in process equipment that are not corroded by chloride ion.
U.S. Pat. No. 3,083,076 discloses the use of an aqueous solution of both sodium sulfate and sodium hydroxide to strip molybdenum and vanadium from the organic solution. Again, this strip is chemically incompatible with the leach and requires corrosion resistant process equipment.
U.S. Pat. Nos. 4,434,140 and 4,434,141 disclose the use of an aqueous solution of bicarbonate anion as being particularly effective for stripping both Group VB and Group VIB metals, particularly vanadium, molybdenum, and tungsten from organic solutions containing quaternary ammonium compounds. Bicarbonate provides high recoveries under mild conditions and is chemically compatible with the ammoniacal leach step in an overall process to recover metal values from spent hydroprocessing catalysts. Hydrochloric acid is added to the aqueous bicarbonate strip solution and ammonium metavanadate is precipitated. The resulting aqueous solution may contain either molybdenum or tungsten or no metals at all. The molybdenum or tungsten can be recovered by reducing the volume of the solution until the metals start to precipitate. The precipitation can be aided by adding an appropriate ion to form a less soluble salt, for example, calcium hydroxide may be added to precipitate the less soluble calcium molybdate.
There is a problem with the use of hydrochloric acid to precipitate out the vanadium metal values since chloride ion is corrosive to process equipment and may cause chemical complications in downstream processes such as incompatible chloride complexes which may not be extractable in solvent extraction.
It has been discovered that vanadium may be precipitated from the aqueous ammonium bicarbonate strip solution by using sulfuric acid and ammonia and/or ammonium hydroxide. This process eliminates the use of chloride ion which is corrosive to the process equipment.